Switching dc/dc converters, whether isolated or non-isolated, have long used a combination of transistors and diodes to implement their switching function. More recently, the diodes have been replaced with transistors called “synchronous rectifiers” for the purpose of reducing the power dissipated by the converter. Typically, MOSFETs are used for the synchronous rectifiers, although other types of transistors such as BJTs and JFETs could also be used. While these transistors can provide a lower on-state voltage than a diode, they do need to be turned on and off at the appropriate times in the switching cycle by the application of a voltage waveform on their “control terminal” (e.g. the gate terminal for a MOSFET). Most transistors (including MOSFETs) can carry current in either direction when they are turned on. Some transistors, such as the MOSFET, also have an anti-parallel body diode inherent in their structure that can carry current when the transistor is turned off. Sometimes a Schottky diode is placed in anti-parallel with the transistor to carry this latter current because it has a lower on-state voltage and a faster turn-off recovery time than the transistor's own body diode. Whether internal or external, this anti-parallel diode will be referred to herein as an “uncontrolled rectifier” to distinguish it from the active part of the transistor (i.e., the channel of a MOSFET), which will be referred to herein as a “controlled rectifier.”
While synchronous rectifiers have been successfully applied in dc/dc converters, a problem arises with their use when two or more dc/dc converters must interact at their output. A dc/dc converter using controlled rectifiers can draw a negative output current, a result that was not possible when only diode rectifiers were used.
For instance, when two dc/dc converters are connected in parallel to provide more output power or redundancy, it is possible for one converter to deliver more output current than the load requires and for the other converter to draw a negative output current to remove the excess. This might typically happen because the first converter wants the output voltage to be higher than does the second converter.
Schemes to enforce current sharing between paralleled converters might solve this problem in the steady state, but they are difficult to make work during “start-up” transients when the converter has been turned on and is switching, but steady-state conditions have not yet reached. They are also difficult to make work during conditions where one or more converter has gone into a current limit or short-circuit protection. Often paralleled dc/dc converters with synchronous rectifiers become oscillatory or have other performance problems under these conditions.
Even with the paralleled converters operating in the steady state, they will not share the load current perfectly. When the total load current is small, one or more dc/dc converters may actually be drawing a negative current. This condition could cause the performance problems mentioned above. At the very least, it results in an inefficient situation where excess power is circulated among the paralleled dc/dc converters.
When redundancy is desired, paralleled converters are often connected at their outputs through diodes so that one failed converter will not bring down the output bus. This “ORing diode” can solve the problem mentioned above because it prevents a converter from drawing a negative output current. However, it is desirable to replace the ORing diode with an “ORing transistor” to reduce its power dissipation. An ORing transistor includes at least a controlled rectifier and may also include an uncontrolled rectifier. Since the controlled rectifier can carry current in both directions when it is turned on, the ORing transistor no longer solves the negative current problem.
Besides paralleled converters, another place where the negative current problem mentioned above comes into play is when connections are made between the outputs of two or more converters to ensure that the difference between their output voltages does not exceed some limit. For example, in a system where both a 5V output converter and a 3.3V output converter are used, it is sometimes desirable to place a “clamp diode” between the 3.3V output and the 5V output to ensure that the 3.3V output never gets more than one diode-drop above the 5V output. Conversely, a chain of three or four clamp diodes in series may be placed between the 5V output and the 3.3V output to ensure the former never gets too high compared to the latter.
If, during start-up or some other transient condition, these clamp diodes become forward biased, then a condition may once again exist in which one converter delivers more output current than is needed by the entire load, and the other converter draws a negative output current. The converters may oscillate or otherwise not work correctly under this condition.
Whether converters are connected together at their outputs directly, through ORing transistors, or through clamp diodes, another condition where the negative current problem can arise is when one of the converters is “shut-down.” This shut-down state may be externally commanded through an ON/OFF control input, or it may be the result of the converter's own protection circuitry sensing an abnormal condition such as a voltage, current, or temperature that is too high. In all such cases, the converter that is shutdown may draw a negative output current from another converter that is holding up the first one's output voltage.
Other conditions not described here may also arise in which a problem is caused by the ability of a dc/dc converter with synchronous rectifiers to draw a negative current.